This invention relates in general to servomechanisms and in particular to a hybrid servomechanism including a speed control, a position control and a brushless motor.
A simple servomechanism employing a brushless dc motor 10 and gear train 12 is shown in FIG. 1. The motor 10 has a permanent magnet rotor which is surrounded by stator windings. Position of the rotor's flux vector with respect to the stator windings is sensed by Hall effect sensors 14. Rotor position signals generated by the Hall effect sensors 14 are processed into commutation commands by decode logic 16. A solid-state inverter 18 commutates current in the stator windings in response to the commutation commands. The energized stator windings generate a stator mmf vector that is displaced 90 .+-.30 electrical degrees from the rotor's flux vector. The resulting torque reaction between rotor flux and stator mmf causes the rotor to rotate.
A closed loop speed control 20 and outer loop 19 are shown controlling motor torque according to a position command CMD, a position feedback signal PFBK and a speed feedback signal SFBK. Generated is an error signal ERR which controls the amount of current flowing through the stator windings. The motor torque is proportional to the product of the motor stator current, the rotor flux and the cosine of the angle between the rotor flux and stator mmf vectors.
The position feedback signal PFBK can be obtained by integrating the speed feedback signal SFBK. Alternatively, the position feedback signal PFBK can be generated by a position sensor 21 which senses the rotation of the gear train's output shaft.
The speed feedback signal SFBK is generated by a motor speed converter 22. Utilizing one or more of the rotor position signals, the motor speed converter 22 measures time between rotor position events to determine motor speed. Utilizing one rotor position signal, motor speed is determined once every 360 electrical degrees. Frequency multipliers utilizing all of the rotor position signals can be employed to increase resolution of the speed feedback signal SFBK.
However, such motor speed converters 22 are known to produce a deadband in the speed feedback signal SFBK. As the motor speed slows, it takes a longer time for the converter 22 to measure rotor events. Thus, accuracy of the speed feedback signal SFBK decreases as motor speed decreases. In the resulting deadband the speed feedback is unusable.
A problem arises when the speed feedback falls within the deadband: the motor 10 cannot hold its commanded position. As the servomechanism reaches its commanded position and the motor speed approaches zero, the servomechanism operates in a stable limit cycle. As a result, the position of the servomechanism oscillates at or near zero speed. Thus, a fixed position cannot be held. Further, the motor movement causes the gear train 12 to wear. The wear is especially rapid when the servomechanism operates at a single position for a large portion of its life.
For applications where the servomechanism is backdriven by an externally-applied load (e.g., the outflow valve of an aircraft cabin pressure control system), the amplitude of the motor movement is amplified, causing increased wear on the gear train 12. Although irreversible gear trains have been used in such applications, they are expensive and difficult to manufacture with repeatable characteristics. In addition, the irreversible gear trains must be made with an efficiency of less than fifty percent, thereby increasing the size and cost of the servomechanism.
The use of a mechanical brake can provide an acceptable means of holding zero speed. However, the mechanical wear on the moving parts of the brake results in frequent maintenance. Further, the brake is expensive, making it undesirable.
Therefore, it is an objective of this invention to ensure that the servomechanism does not backdrive when a constant position is being held.
It is a further objective to eliminate the limit cycle operation of the servomechanism at zero speed command, thereby eliminating the otherwise rapid wear of the gear train 12.